Objects such as printed wiring boards containing two-dimensional images require time-consuming inspection to provide quality control. Manual inspection is tedious and of varying accuracy. Presently, computer-aided machine inspection requires tremendous data processing ability: digital representation of a two-dimensional image presents substantial and often insurmountable computational difficulties.
Copper circuits of varying complexity are normally printed over a 16.times.22 inch area of a printed wiring board and can be depicted as a pattern of solid lines. The lines vary in width and are positioned at differing locations over the printing wiring board. Improper printing of the lines renders a circuit inoperative. There is a need for machines which can rapidly compare the actual printed patterns with predetermined, desired patterns.
Geometrically, a solid two-dimensional line having a known width can be represented using differing amounts and types of information. Generally, methods using fewer bytes of information are preferred for computer processing. One method identifies the beginning x,y coordinate, the end x,y coordinate, and the width perpendicular to the one-dimensional line defined by the beginning and end coordinates. This method therefore requires five numbers: four for the coordinates and one for width. Another method applies a vector format to represent a solid line. A vector having a beginning x,y point is defined in length, direction, and width. Five numbers are also used in the vector method: two numbers for the beginning coordinate and three for the dimensions of the solid line. Alternatively, a solid line may be represented using eight coordinates to define the four corners of line.
Although these representative methods allow succinct notation of a two-dimensional image, each such 37 compression" of the image requires conversion during re-creation or generation of the image in its entirety. These compression methods presently do not readily lend themselves to the raster format of sequential line-by-line observation or generation of a two-dimensional image. For large, complex images such as a 16.times.22 inch printed wiring board, present representational methods are not economically feasible.
Typically a software program must convert image information into this raster format to enable a conventional CRT screen to display the image. Along each raster line, that is, a horizontal row of pixels, the program designates whether the raster line intercepts the image and, if so, where to begin and end activating the pixels necessary to display the image. Using the compressed image information, the program must identify for each raster line the beginning pixel and the end pixel or the beginning pixel and the number of pixels following that pixel. Presently, this identification process is relatively slow and time-consuming; its limitation becomes prohibitive for rapid depiction of printed wiring boards.